The changing face of finals footy and the numbers behind it

Competition in our footy codes drew to a close over the weekend, with Penrith claiming a historic fourth premiership in a row in the NRL grand final. It followed the conclusion of the AFL the week prior, when Brisbane had an emphatic grand final win over Sydney.

Brisbane came from the clouds to win from fifth position on the AFL ladder over the minor premiers. Since the inception of the current AFL final eight system in 2000, this was only the second time that a team has won from outside the top four. The Western Bulldogs won it from seventh in 2016 (with again, the hapless Sydney minor premiers and runners-up).

This got me thinking about the overall performance of top four sides in recent times, where inglorious finals exits seem more prevalent. In particular, I wondered what effect the controversial pre-finals bye introduced in 2016 has had on top four sides’ chances. I had a feeling that teams 1 to 4 were losing continuity in playing, particularly for those teams that won in the first week of the finals, advancing straight through to the third week. Meanwhile, the bye would perhaps favour teams with final ladder positions 5 to 8, enabling them to freshen up before week one of the finals, reset, and build momentum with every final sudden death for them.

First, a quick look at how the AFL final eight system works:

Week 1
Qualifying Final 1: 1st final ladder position vs 4th final ladder position
Qualifying Final 2: 2nd vs 3^rd
Elimination Final 1: 5th vs 8^th
Elimination Final 2: 6th vs 7^th
Week 2
Semi-final 1: loser QF1 vs winner EF1
Semi-final 2: loser QF2 vs winner EF2

Week 3
Preliminary-final 1: winner QF1 vs winner SF2
Preliminary-final 2: winner QF2 vs winner SF1

Week 4
Grand-final: winner PF1 vs winner PF2

Source: AFL.com.au

I looked at two measures to probe my intuition, over the periods 2000 to 2015 when there was no pre-finals bye, and post 2015 when it was introduced:

1. The frequency with which top four teams won their first final, then exited in their subsequent week 3 preliminary final. So those teams bypass week 2 finals, and in the pre-finals bye period, may have only played one match in nearly four weeks up until preliminary final day.
   
   
2. The frequency with which top four teams exited the finals in so-called ‘straight sets’. That is, a loss in weeks one and two of the finals series.

The results were compelling.

From 2000 to 2015, only four times did top four teams lose the preliminary final having won their week 1 final and bypassing week 2. That is, just 12.50% of the time (4 out of 32 losing preliminary finalists). From 2016 to 2024, seven top four preliminary finalists failed to advance to the grand final after a first week win. That is, a strike rate of 43.75% (7 out of 16 losing preliminary finalists). Note, 2021 has been excluded from the analysis, when there was no pre-finals bye due to a Covid restructured season. 

That’s a significant increase in rate of failure to advance, which supports the theory that top four sides were losing continuity in game time, while the lower ranked teams had momentum on their side.

Straight set exits have also spiked

For the period 2000 to 2015, there were five straight set exits out of a total of 64 top four finals participants, a rate of just 7.81%. For 2016 to 2024, that rate jumped to 21.88%, as seven out of 32 teams lost in rounds one and two of finals. 

Again, this backs the assertion that teams ranked 5th to 8th have closed the gap on the top four. Perhaps because the week off has provided a circuit breaker between a long season and an arduous finals series, where the teams need a win every week to progress. Without the bye, there would be no respite for those teams.

The pre-finals bye was introduced by the AFL in 2016. By that point, there had been several cases of teams that had already locked in a certain finals position resting a number of players to keep them fresh for the upcoming finals. The AFL thought this compromised the integrity of the competition as clubs were not fielding their strongest available teams. In theory, a pre-finals bye meant that teams would not need to sideline players in the last round.

But did the ruling have unintended consequences? The analysis here suggests that it did. The AFL may be comfortable with that as it seems to have brought about a less predictable finals series. Others would argue that it unfairly disadvantages higher ranked teams, who have worked hard over a long season to achieve top four status and the week one double chance.

Note that this year, minor premiers Sydney did not fall into the two categories analysed here, instead winning in weeks 1 and 3 before losing the grand final. However, by the day of the preliminary final, Swans had played only one game in 27 days. Meanwhile, all-conquering Brisbane had played four hard-fought finals in as many weeks, building up significant momentum in the process. Did a lack of match practice - aided by the pre-finals bye - bring about Sydney’s demise?

Footnote

For those interested, assuming each team has a 50% chance of winning (or losing) per final, the probabilities of teams ranked 1 to 4, and 5 to 8 winning the premiership, can be calculated at the outset of the AFL final eight series.

Teams 1 to 4 can become premiers via one of two paths:

1. Win weeks 1, 3, and 4 (the path most frequent, with 17 premierships since 2000).
2. Lose week 1, win weeks 2, 3, and 4 (6 premierships).

Therefore, the probability teams 1 to 4 win the premiership = (0.5 x 0.5 x 0.5) + (0.5 x 0.5 x 0.5 x 0.5) = 18.75% each.

Teams 5 to 8 can only become premiers via one path: win weeks 1, 2, 3, and 4 (happened only twice). Therefore, the probability teams 5 to 8 win the premiership = (0.5 x 0.5 x 0.5 x 0.5) = 6.25% each.

Note that these numbers are more about relativities than absolute probabilities because in reality, a probability other than 50% of winning per final would be arrived at by factoring in characteristics like recent form, home ground advantage, injuries, and so on.

The numbers show that teams 1 to 4’s chances of winning the grand final, are possibly several multiples of that of 5 to 8’s chances. Which emphasises the difficulty of winning from outside the top four, and puts Brisbane’s effort into perspective.

Tony Dillon is a freelance writer and former actuary. This article is general information and does not consider the circumstances of any investor.